I've done the footwork on this one (and don't believe what the suppliers tell you - they lie!), and really it's just a gadget until you swap to a 100% provider. It takes one phone call, and I can guarantee that at Ecotricity it will be a real person you talk to.

The other providers Greenwash horribly, but the statistics are clear - it's p*ssing in the wind with any of their "green tarrifs". Use one of the two good ones - Ecotricity or Good Energy.

I'd disagree with the 'it's just a gadget until...'. Yes it is if all you do is sit there and count the pennies rolling in from the feed-in tariff. However, part of the reason for being a monitoring geek about it is so that we can detect when we've got power to spare and *use* it for things that we'd otherwise run in an evening, rather than just let it feed back to the grid.

I read the other blog, and it is power factor to which you seem to be referring. If you multiply the instantaneous voltage and current continually over the mains cycle, you can calculate the power even if the current and voltage are not exactly in phase, and/or if the current waveform does not match the voltage.

Motors often have a low power factor because of inductive phase shift, as do fluorescent lights. Older switchmode power supplies, in televisions and computers (or low power ones that do not have to be power factor corrected by law), and most low power lamps, have a low power factor because most of the current is drawn near the peak of the voltage waveform. So bad is this effect that the mains sinewave can normally be seen to be flattened at the top.

Incandescent lamps and heating elements pretty much have unity power factor, so for those, measuring the RMS voltage and the RMS current, and multiplying them, tells you the power.

I reckon that with an Arduino and a wireless shield, programming it in C not Arduino sketch on account of speed, it should be possible to make something that measures current and voltage many times per mains cycle and calculates the power *and power flow direction* then sends it out via bluetooth. I am not yet a good enough realtime programmer to do that, however.

I don't think there is much green power in the UK - in the sense of renewable energy, though if carbon dioxide is going to cause the problems that are claimed in the next hundred years, nuclear power is clearly better than burning fossil fuels. Solar energy is renewable, and produces several times as much energy during its working life as it takes to manufacture it, even if it can't keep the lights on at night. For wind power, the balance is in the other direction, even if there may (and I am not sure about this) be one or two wind power farms that are likely to generate slightly more in their working lives that it took to manufacture and maintain them, the UK average is definitely "anti-green" for wind power at present. In principle wind power could be made ecologically sound, but it is an inherently difficult problem, partly because the power output is proportional to the cube of the wind speed, and it is difficult to make generators efficient at ordinary wind speeds and not destroyed by higher ones. At the least they have to shut down if the wind is too powerful. Extremely powerful magnets, using "rare earth" materials which are - well - rare, are needed for useful efficiency. Substitutes using more complex materials made from more abundant ingredients are being developed, but even so, I doubt that we shall see a wind power station that delivers, say, ten times as much energy as it takes to build and maintain it, in the foreseeable future. I think that solar power passed that level years ago.

If you live in Scotland, there is a fair amount of hydroelectric power, which is surely "green" in the renewable sense. However, I think that hydroelectricity does not generate as much electricity as Scotland uses, so there is no greenness to buying Scottish hydroelectricity elsewhere, reduced by transmission losses.

As always, someone's already done it http://openenergymonitor.org/emon/I've just been building one myself with the intention of adding code to give me a voltage output proportional to the excess generation, and using that to drive a triac circuit feeding my immersion heater(*). Not as direct as solar thermal, but also quite a bit cheaper than a new solar thermal install, and should bring me a bit closer to the nominal 50% export that the energy companies assume.

(*) Yes, a triac will draw full power for part of the cycle and nothing for the rest, but apparently electricity meters average over each (half) cycle, so don't see this as import. I have yet to add that bit to confirm it.

It looks a good project, but it apparently doesn't account for power factors. It simply measures currents - maybe RMS, maybe "peak scaled RMS" and subtracts one from the other. It will be as accurate as the power factor of the whole load, and a phase controlled heating element does not have a great power factor.

If you only switched on the immersion heater when the export power is equal to, or greater than, the heating element load, and switched if off if more than (say) 500W was being imported, that would probably save a little money on hot water - though less so with the water heating wossname already being there. If there is spare solar electricity, there will also be plenty of direct heating.

OEM has an option to add an AC voltage input (mains dropped to 9V via a transformer), at which point it does read real power, apparent power, and power factor. It samples I & V at about 2.5kHz, and does compute genuine RMS based on those values.

If you're just trying to make the best use of the electricity generated by the PV system, then my understanding is that it doesn't matter too much as to the power factor. Depends on exactly what the supplier's meter is measuring, but they do quite a bit of averaging already.

A 3kW immersion heater is unlikely to kick in if we just switch it when the PV is producing sufficient output (or even 500W less). Almost all systems for FITs will be under 4kW, so it'll only be a few months of the year that you get sufficient generation. Our system is only 3.84kW total, and I have no solar thermal, so adding control circuitry is the cheapest way to get some benefit. I could change the immersion heater to a 1kW one, but then the tank will take longer to heat when I need to boost it.

"I could change the immersion heater to a 1kW one, but then the tank will take longer to heat when I need to boost it."

Dual element tank recommended, assuming you actually have a need for that big a tank. (I don't: No bathtub in this place. =:o} ) The usual arrangement is a 2kW or 3kW element at the bottom for heating the entire tankful when required (i.e. bathnight), plus a 1kW element at about 1/3rd of the way down, to top-up just the upper portion for casual daytime use. Traditional wiring is to have the lower element run by overnight "cheap" 'lecky and the upper one on a manual switch for when you want to boost it up, but you can also use it the other way round with the booster switch being "I want a bath later today", while the upper element runs (a) overnight (using at most a third as much energy as heating the whole tank), plus (b) whenever there's spare solar power for it.